Epoxy compositions are formed, at a minimum, from an epoxy resin and a hardener. The epoxy resin can include one or more epoxide groups, while the hardener can include a variety of structures that react with the epoxide group. These two components chemically react, or “cure,” to form a cured epoxy product. Upon curing, the epoxy composition forms a highly cross-linked thermoset.
Cured epoxy compositions can have a variety of properties. These properties can depend upon the choice of epoxy resin, hardener, ratio of components, reaction conditions and additives, if any, present in the epoxy composition. An example of one such additive is a liquid toughener, which can be added to the epoxy composition in order to improve the toughness of the cured epoxy composition. One of the most common liquid tougheners is carboxyl-terminated butadiene-acrylonitrile copolymer (CTBN) adducted with bisphenol A diglycidyl ether. Some shortcomings of the CTBN adduct, however, include its very high liquid viscosity and its immiscibility in liquid epoxy resin. The immiscibility, in turn, lead to the cured epoxy resin being opaque due to the formation of large CTBN particles in the cured epoxy composition.
A need therefore remains for a toughening agent for epoxy compositions that is both low in viscosity and that will not adversely affect the optical properties of the cured epoxy composition.